hw/nvme: Avoid dynamic stack allocation

Instead of using a variable-length array in nvme_map_prp(),
allocate on the stack with a g_autofree pointer.

The codebase has very few VLAs, and if we can get rid of them all we
can make the compiler error on new additions.  This is a defensive
measure against security bugs where an on-stack dynamic allocation
isn't correctly size-checked (e.g.  CVE-2021-3527).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>

hw/nvme/ctrl.c

@@ -894,7 +894,7 @@ static uint16_t nvme_map_prp(NvmeCtrl *n, NvmeSg *sg, uint64_t prp1,
     len -= trans_len;
     if (len) {
         if (len > n->page_size) {
-            uint64_t prp_list[n->max_prp_ents];
+            g_autofree uint64_t *prp_list = g_new(uint64_t, n->max_prp_ents);
             uint32_t nents, prp_trans;
             int i = 0;